BEFORE you stock, there are some things that you should consider. These are very important.
Firstly, what is your water like? Is it hard or soft, and is your pH high or low? This will effect the kind of fish that will be able to live long, healthy live in your water. Tetras and danios prefer softer water with a low pH. Mollies like hard water with a high pH.
Secondly, I have to agree with Lebron on the mollies. They are really fish that need 20 gallons. They are not great betta tankmates, IMO, as they won't thrive in freshwater the way they do in brackish water, which your betta would hate. They also prefer lower temperatures.
Thirdly, danios are a poor choice also. They are sub-tropical fish, where as bettas are high-end tropical (i.e. bettas need water in the 78-82 range to thrive, whereas danios do best in temperatures below 75, with 72 being great). Danios are also super active and need the full length of a 2 foot tank. If you got the 15 galon you mentioned earlier, it just doesn't give them a large enough footprint to swim in.
Neons are probably a better idea, but they are still a fish that enjoy zoom room. If you have water soft enough for tetras, consider ember tetras. They are smaller and less active than neons, so should be fine in the footprint of your tank.
I'm sorry that this limits your options, but it is really important to make sure that the fish you get will be happy in your tank. :) Congrats on the guinea pigs, and make sure you find out hardness and pH!